The invention relates to a process for separating and preparing paraxylene from a mixture of aromatic hydrocarbons comprising xylene isomers.
The xylene isomers are orthoxylene, metaxylene, paraxylene and ethylbenzene. The main application of the invention is in obtaining paraxylene to a degree of purity that is sufficient for the synthesis, for example, of terephthalic acid, used in the preparation of synthetic fibers and particularly polyester.
The applicant has described in patent FR 2,681,066 (U.S. Pat. No. 5,284,992), which is incorporated as a reference, a process for separating and recovering paraxylene contained in a charge of hydrocarbons comprising essentially aromatic hydrocarbons having 8 carbon atoms.
This process comprises the combination of an enrichment step, and a purification, the enrichment is a selective adsorption on an adsorbent in a simulated moving bed of a charge containing essentially the xylene isomers, which makes it possible to substantially enrich a first adsorption effluent with paraxylene. The purification which is a crystallization, in at least one crystallization unit operating at high temperature, of the paraxylene-enriched effluent so as to obtain paraxylene having a very high purity, the mother liquor being recycled to the adsorption step.
This high temperature crystallization corresponds to the second crystallization step of conventional crystallization processes (Chevron; Arco) which generally comprise a first crystallization at low temperature (-40 to -70.degree. C.) and a second crystallization to purify, at high temperature (0 to -20.degree. C., for example), crystals obtained that were previously remelted.
Furthermore, a second fraction depleted of paraxylene and therefore enriched with ortho- and meta-xylene as well as ethylbenzene, delivered by the selective adsorption unit, is sent into an isomerization unit to increase its concentration of paraxylene to a value close to equilibrium and approximately close to or greater than the composition of the initial charge of hydrocarbons, and the isomerate obtained can then be recycled to the adsorption.
In the adsorption, crystallization and isomerization sequence described, various types of impurities can appear in the various effluents and cause disturbances in the operation of the units, which are detrimental to the yield obtained and to the purity of the paraxylene recovered.
First of all, during isomerization of the paraxylene-depleted fraction, olefinic hydrocarbons can be produced in a variable amount depending on the values of the partial pressures of hydrogen introduced. The subsequent formation of polymers and their passage into the adsorption unit can cause serious problems of circulation through the adsorbent, even deactivate it.
In addition, paraffinic and naphthenic hydrocarbons having 8 and 9 carbon atoms, whose volatility is between that of toluene, a desorption solvent for example, and that of the xylenes, are intermediate products of the conversion of ethylbenzene into xylenes during the isomerization and their accumulation can prove to be harmful.
Furthermore, aromatic hydrocarbons having 9 carbon atoms, present in low proportion and poorly separated in distillation columns, can be detrimental to the process. Such hydrocarbons include aldehydes and ketones that are heavier than the initial charge, which are formed when oxygen is accidentally dissolved.
Finally, another problem is linked to the presence of methanol. This alcohol is sometimes added in low proportion in mixtures of xylenes to be crystallized to prevent the cocrystallization of water and paraxylene.
Actually, the dry C.sub.8 -aromatic mixtures are relatively hygroscopic and when the suspension of paraxylene crystals in the mother liquor passes into the centrifuges, water contained in the ambient air can be absorbed into the mother liquor and this water can possibly crystallize in conjunction with the temperature of this mother liquor.